Effect of Escrow Funding on the Company Repurchasing its Common Stock (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Dec. 31, 2011		Mar. 31, 2012
Stockholders Equity Note [Line Items]
Deposits under the retrospective responsibility plan			$ 1,565
Effective price per share (in dollars per share)	$ 101.75	[1]
Class A common stock
Stockholders Equity Note [Line Items]
Equivalent shares of class A common stock effectively repurchased	15.4
Conversion rate of class B common stock to class A common stock after deposits
Class B common stock
Stockholders Equity Note [Line Items]
Conversion rate of class B common stock to class A common stock after deposits	0.4254		0.4254
As-converted class B common stock outstanding after deposits	104
Deposit into Litigation Escrow
Stockholders Equity Note [Line Items]
Deposits under the retrospective responsibility plan			$ 1,565

[1] Effective price per share calculated using the volume-weighted average price of the Company's class A common stock over a pricing period in accordance with the Company's amended and restated certificate of incorporation.